(1. China Geo University (Beijing); 2. Langfang Branch of China Petroleum Exploration and Development Research Institute)

About the author: Wang Yibing, born in 1966, is a senior engineer who has been engaged in coalbed methane exploration and research for a long time. He is a Ph.D. student in the School of Energy, China Geo University (Beijing), and his address is Room (065007)44, Langfang, Hebei.

Ningwu basin is a small structural residual basin in the northwest of Shanxi Province. Carboniferous-Permian coal seams are developed in this basin, and the coal rank is mainly middle rank gas, fertilizer and coking coal, which is the most favorable coal rank for coalbed methane exploration. The coal seams at both ends and wings of the basin are moderately buried and have the conditions for coalbed methane accumulation. However, due to the complex structural and hydrodynamic conditions, the variation law of gas-bearing gas production in coal seams is complex. On the basis of analyzing and summarizing the exploration history, exploration achievements and enlightenment, the gas control conditions of coalbed methane in the basin are re-recognized, the favorable reservoir-forming ranges of coalbed methane in the north and south ends of the basin and the deep parts of the east and west wings are selected, and the favorable areas for coalbed methane exploration in the next step are determined.

Keywords: favorable conditions and targets of coalbed methane enrichment in Ningwu Basin

Enlightenment from the analysis of coalbed methane exploration effect in Ningwu basin

Wang Yibing 1, 2, Sun Ping 1, Xian Baoan 2.

(1. China Geo University, Beijing100083; 2. Langfang Branch of China Petroleum Exploration and Development Research Institute, Langfang 065007)

Abstract: Ningwu basin is a small structural residual basin in northwest Shanxi. Permian-Carboniferous coal seams are developed, and the coal rank is medium volatile bituminous coal, which is beneficial to coalbed methane exploration. The buried depth of coal seams at both ends and wings of the basin is moderate, and the conditions for coalbed methane accumulation are mature. However, due to faults and hydrogeology, the gas content and production potential of coalbed methane change complicated. On the basis of summarizing the exploration history, exploration achievements and experience, this paper discusses the controlling factors of coalbed methane enrichment in this basin, identifies favorable coalbed methane enrichment areas at the north and south ends of the basin and the east and west wings, and selects the future exploration areas.

Key words: Ningwu basin; CBM enrichment conditions; Target area

1 regional geological background

Ningwu Basin is located between Luliangshan Uplift and Wutaishan Uplift, with Luyashan anticlinorium in the west and Wutaishan Uplift in the east. It is an intermountain tectonic basin with northeast-trending banded distribution formed by multi-stage tectonic movement after the late Paleozoic coal-forming period. It belongs to the south section of Datong-Ningwu coal-bearing area in the northwest of Shanxi Province, with a length of about 130km from north to south, a width of about 20-30km from east to west and an area of about 365,438+.

Figure 1 Buried Depth Map of Main Coal Seam of Taiyuan Formation in Ningwu Basin

Carboniferous-Permian and Jurassic coal-bearing strata developed in the basin. Carboniferous-Permian coal-bearing strata are mainly Upper Carboniferous Taiyuan Formation (C3t) and Lower Permian Shanxi Formation (P 1s), with 6 ~ 10 coal-bearing layers, which are concentrated in 80 ~ 100 m interval, among which 4 # coal at the bottom of Shanxi Formation and 9 # coal at the bottom of Taiyuan Formation are thicker. Jurassic has poor coal-bearing property and small distribution range, which is the target layer for secondary exploration.

The late Paleozoic Taiyuan period in this area is a low-level swamp of coastal and shallow-sea delta plain subfacies under the background of regression, and its provenance mainly comes from the ancient land north of Datong. In the early stage, the plants were lush and the peat was thick. 9 # coal is deposited in the lower part of Taiyuan Formation, with a thickness of 4.36 ~ 24.62 m and an average thickness greater than11m.. In the middle and late stage, the seawater retreated southward, the water body became shallower and shallower, and the coal-forming environment became worse. Only some discontinuous thin coal seams and littoral mudstone-marl are developed in the middle and upper part of Taiyuan Formation. During Shanxi period, the seawater was exhausted, the initial sedimentary environment was stable, and it was in the swamp sedimentary microfacies area of river subfacies. In the lower member of Shanxi Formation, a thick 4 # coal was formed, with a thickness of 0.37 ~ 13. 15m, generally 2 ~ 4m. In the middle and late period, the swamp facies belt contracted, and the coal-forming conditions became worse, and a thin coal seam with unstable lateral distribution of 1 ~ 3 was deposited, with a single layer thickness of 0 ~1.95 m.

After the coal-forming period, the area mainly experienced the tectonic movements of Yanshan period and Himalayan period. During Yanshan period (early Jurassic to late Cretaceous), the strong torsional compression formed the NE-trending faults and fold systems, and formed the embryonic form of the intermountain basin in Ningwu Basin, which basically controlled the overall structural pattern of the basin. The NW-SE tensile stress caused by the Himalayan intracontinental plate movement transformed the early structural features and formed a series of structural features dominated by NE-trending extensional faults. At present, the structural form is a syncline structure spreading to the northeast, with steep wings and dip angle of 25 ~ 40, and the structures at the north and south ends are relatively gentle with dip angle of 6 ~ 12.

The buried depth of coal seam is mainly controlled by structure and topography, and the change law is simple. Generally speaking, the edge is shallow and the abdomen is deep, and the local area is slightly affected by faults. The buried depth of coal seams at the north and south ends and wings is generally less than 1500m, and the deepest part of abdomen is about 2700m.

2 CBM exploration history

Geological exploration in Ningwu basin began in 1950s. In the early stage,1:10000 ~1:200000 regional geological survey was mainly carried out. In 1970s and 1980s, a coalfield survey was carried out in the shallow part around the basin, the depth of which was less than 600 m m, and a detailed coalfield survey was carried out in some areas, and the development law of coal seams around the basin and the change law of coal and rock properties were preliminarily mastered. CBM exploration can be divided into two exploration stages.

2. 1 Phase I: regional exploration phase

According to the data of coal exploration, the structural conditions, coal quality, evolution law, roof and floor sealing conditions and hydrodynamic conditions of the basin are comprehensively analyzed, and it is considered that No.4 coal and No.9 coal in Ningwu Basin are the main exploration targets of Shanxi Formation and Taiyuan Formation respectively. Planarly, ningwu county is dominated by low metamorphic coal in the north, and ningwu county is dominated by medium metamorphic coal in the south, mainly gas fertilizer, chemical fertilizer and coking coal. Endogenous cracks generally develop. Vertically, from 4 # coal to 10 # coal, with the deepening of burial, the degree of coal metamorphism increases and the coal rank increases; The methane gas content in coal seam is 8.84 ~ 25.89 m3/t according to the mine gas emission. At the north and south ends of the basin, the areas where coal seams are buried below 1000 m and 1500 m are relatively open, covering an area of more than 500km2. The area with simple structure has the conditions for coalbed methane accumulation and is a favorable area for coalbed methane exploration and development.

Wushi 1 well is the first coalbed methane exploration well in the basin, which is located in the gentle slope zone in the south of the basin. The buried depth of main coal seam is 826.2 ~ 9 12.2 m, the thickness of 4 # coal in main coal seam is 3m, and the thickness of 9 # coal is 14m (Figure 2). The macroscopic types of coal and rock are mainly bright-semi-bright coal, followed by semi-dark coal, with low ash content and medium volatile matter. The maceral is mainly vitrinite, with an average content of more than 60%, but it changes greatly in the longitudinal direction, especially in No.9 coal, which is obviously formed by the superposition of several consecutive coal-forming periods, reflecting that the sedimentary environment changes greatly during the coal-forming period, and the vitrinite content changes with the coal-forming swamp. The coal seam is medium-rank fat coal, ro:1.0%; The gas content in the main 9 # coal seam is 9.76 ~ 13.97m3/t, with an average of 12. 1 1m3/t, and the adsorption saturation is 86.29% (Figure 3). Well test permeability is 0.86× 10-3μm2, average daily water production after fracturing 150m3, daily gas production 1 120 ~ 2476m3, maximum daily gas production 31kloc-0/2m3, and formation water mineralization.

Fig. 2 Comprehensive logging map of Wushi 1 well.

Fig. 3 Isothermal adsorption curve of Wushi 1 Well 9 # coal

The following conclusions and understandings can be drawn from the drilling of 1 well:

(1) The distribution of coal seams in the deep part of the basin is stable, and the coal and rock properties and the content of organic components have little change.

(2) The evolution of coal seam is dominated by compaction metamorphism, with a moderate degree of evolution. The coal rank is gas-fat coal, and the evolution degree becomes higher with the increase of buried depth. Ro at the edge of the basin is 0.8%, and the buried depth of 900m increases to 1.0%. The coal seam has entered the peak of gas generation, and the gas source is sufficient.

(3) The adsorption saturation of coalbed methane is high and the desorption pressure difference is small, which is beneficial to depressurization and desorption of coalbed methane in the drainage stage. The original pressure of coal seam is as high as 8.7MPa, the desorption pressure is 4.7MPa and the desorption pressure difference is only 4MPa (Figure 3).

(4) The physical properties of coal seam are poor, but it is in the best coal rank to preserve cleavage cracks in coal seam. Coal and rock have good mechanical stability, and fracturing can achieve good results.

(5) The south slope of the basin has good conditions for coalbed methane accumulation, and there is stagnant water area with high salinity, which has great prospects for coalbed methane exploration; It is speculated that there may be high gas-bearing areas with similar coalfield geological conditions in the northern slope of the basin, and it is expected to find "small but fat" coalbed methane fields, which is a favorable area for coalbed methane exploration.

2.2 Stage II: Pre-exploration stage

Well Wushi 2, Well Wushi 3 and Well Wushi 4 were abandoned from shallow to deep on the south slope, and Well Wushi 7 was drilled on the north slope.

Three wells in the south are evenly distributed on the slope, with a distance of 4 ~ 8 km, and the buried depth of the main coal seam is 450 ~ 14 10m. Borehole proved that the distribution of main coal seams was stable, with the thickness of 4 # coal being 2.2~4.0m, and the thickness of 9 # coal being10.6 ~1.8m. The type of coal and rock, ash and organic composition did not change much, but the evolution degree and gas-bearing property changed greatly with the structural position and buried depth. When the depth is 1400m, the Ro reaches 1.40%, and the coal rank is coking coal. The gas content in shallow layer is only 3.4 ~ 5.0m3/t, and gradually increases to 14 ~ 16m3/t in deep layer, and the saturation increases from 50% to 80% ~ 90%.

The 9 # coal test and post-fracturing gas test of Wushi 3 and Wushi 4 wells show that the water production, gas production and formation water properties of each well are quite different (table 1). The formation pressure gradient of Well Wushi-3 is 0.8 1MPa/ 100m, the coal seam permeability is 0.08× 10-3μm2, the daily water production after fracturing is as high as 4 16m3, the salinity of formation water is only 985.7mg/L, and the liquid level fails to drop to the coal seam for desorption. The formation pressure gradient of Well Wushi 4 is 1.0MPa/ 100m, the permeability is1.06x10-3μ m2, the daily water production after fracturing is 15.5m, the maximum daily gas production is 939m3, and the formation water salinity is 3072. Well Wushi 7 is located in the west wing of the north slope, with the main coal seam spacing of 905 ~ 985 m, in which 4 # coal is composed of 3 layers with a thickness of 9. 1m and 9 # coal with a thickness of16.1m. The coal rank is fat coal, RO: 1.02%, gas content is 4.2m3/t, and adsorption saturation is 58.63%. Because of the low gas content and adsorption saturation, gas testing was not carried out in this well.

Through the exploration of the slopes at the north and south ends of the basin, we have the following understandings and conclusions about the exploration of coalbed methane in the basin:

(1) Gas content is closely related to the degree of coal seam metamorphism. With the increase of buried depth and metamorphic degree, the gas content increases (table 1).

Table 1 Statistical Table of Drilling Results of No.9 Coal in Ningwu Basin

(2) With the increase of buried depth and coal rank, the physical properties of coal become worse. Wushi 1 well coal seam buried depth 1400 m, and the gas and water production after fracturing are very low.

(3) The enrichment of coalbed methane is closely related to the development of faults, and the gas-bearing property of coal seams near faults is obviously worse. The influence of faults on the accumulation and production of coalbed methane is mainly manifested in two aspects: on the one hand, the opening of faults leads to the poor sealing of coalbed methane reservoirs, the reduction of coal seam pressure and the direct overflow of coalbed methane through faults; On the other hand, faults connect the upper and lower aquifers, and hydraulic alternation leads to the loss of coalbed methane. The main reason for the poor gas-bearing properties of Well Wushi 2 and Well Wushi 7 is that the slope edge is severely cut by a series of faults distributed in the northeast (Figure 4). The formation and distribution of these faults are related to the tensile stress in Himalayan period, and they are generally not closed, which is not conducive to the preservation of coalbed methane.

(4) The salinity of formation water in Wushi 3 well is low, and the salinity of formation water in Wushi 1 well and Wushi 4 well is high, which proves that the dynamic conditions of groundwater in different parts of the slope are complex, which not only develops a stagnant groundwater environment that is conducive to coalbed methane preservation, but also exists a low salinity groundwater exchange zone that is not conducive to coalbed methane preservation.

(5) The Wushi 1- Wushi-4 well area is structurally stable, with undeveloped faults, and the salinity of coal seam water reaches 2923 ~ 3072 mg/L respectively, indicating that the coal-bearing formation water is in a stagnant or weak runoff area in a closed environment, which is beneficial to the preservation of coalbed methane. At the same time, the large output of coal seam water effectively reduces the coal seam pressure in the drainage stage, and the adsorbed gas can be desorbed in large quantities, forming a high gas production capacity. Based on this, it can be inferred that the structures in the middle-eastern part of the south slope and the deep part of the north slope and the deep part of the two wings of the basin are relatively stable, the faults are undeveloped, and they are in the groundwater blocking or weak runoff area, and the CBM preservation conditions are good, which can be used as favorable targets for CBM exploration in the next step.

3 Enlightenment and suggestions

3. 1 Explore the enlightenment

(1) It is correct to take the study of regional coal seam geology as the guide, the study of coalbed methane accumulation conditions and enrichment law as the core, and the favorable target area for coalbed methane exploration as the focus. The exploration potential of middle rank coalbed methane is huge.

(2) The control factors of coalbed methane in Ningwu Basin are complex, and the preservation conditions and hydrodynamic conditions of coal seams in different structural parts are complex, and the permeability changes greatly, which leads to great differences in gas-bearing and gas-producing capacities of coal seams, which must be paid great attention to in the next exploration.

(3) In terms of specific exploration technology, the permeability of coal seams in Ningwu Basin is generally poor, and the permeability is generally only (0.08 ~1.08) ×10-3 μ m2, so the gas production of coal seams in vertical well fracturing coal mining method is low.

3.2 Exploration ideas and suggestions

Exploration has confirmed that the coalbed methane in the structurally stable area with undeveloped faults has good preservation conditions and high gas-bearing capacity, but the output drops rapidly. It is suggested to carry out the following work:

(1) Strengthen the in-depth study of basin structural conditions, confirm the structural background of the target area, vigorously carry out and strengthen geophysical exploration in specific tactical actions, and confirm the distribution of coal seams, structural forms and faults, especially the distribution and combination relationship of faults, through low-cost non-seismic means, such as magnetotelluric sounding, and study their influence on preservation conditions;

(2) Strengthen hydrogeological research and accurately grasp the control effect of hydrological conditions on coalbed methane enrichment;

(3) In view of the low permeability of coal seam in the basin and the poor development effect of vertical well fracturing, on the basis of fully summarizing the development technology of M 1- 1 coalbed methane pinnate horizontal well, drilling along high-steep coal seam is carried out in the steep parts of the two wings of the basin to improve the single well production and expand the exploration field of coalbed methane.

refer to

[1] Li Wenyang et al, exploration and development of coalbed methane in China in 2003. Xuzhou: Published by China University of Mining and Technology.

[2] Qian Kai et al. 1996. Exploration and development theory and experimental testing technology of methane gas in coal seam. Beijing: Petroleum Industry Press.

[3] Huang Jingcheng et al. 1990. Coalbed methane translation set. Zhengzhou: Henan Science and Technology Publishing House

Han Dexin et al. 1996. Coal petrology in China. Xuzhou: China University of Mining and Technology.

[5] General Administration of Coalfield Geology of China. 1993. Map of key coal development and construction mining areas in China (internal data)